This is MATH (not ethics/morality). I suggest you try it.
The fundamental equation is C = PER x P where C is total consumption, PER is consumption per person, and P the population.
Consider the population is in quintiles by age, 0-20, 20-40 (childbearing if women), 40-60, 60-80, and 80-100. Label these Q1, Q2, Q3, Q4, and Q5. At the start, assume each of these P/5 (over simplified in actuarial terms to make your math easier.)
Each generation assume births of birth rate x Q2/2 (half of Q2 women.
Your suggest, reduce PER, lets say cut it in half (say we think could survive on that. Initially looks great, we have cut total consumption in half, but try running generations for a birth rate > 2 Let's try 3.0
In each generation there are births and deaths and people move up to the next quintile. So births = Q2/2 X 3. We have Q5 deaths, then move up so new Q5 = Q4, new Q4 = Q3, new Q3 = Q2, new Q2 = Q1, and Q1 = births. We add Q1 + Q2 +Q3 +Q4 + Q5 to get the new P and recompute the new total consumption C (we have already reduced PER as much as we can and still survive -- you can choose a fraction for survival that is less, it won't change the eventual outcome).
How many generations before C is now greater than what we started wit? Twice what we started with? Ten times what we started with.? Don''t you understand that if B > 2 P will increase EXPONENTIALLY.